1. Field of the Invention
The present invention relates to a method of inspecting whether or not a pixel portion is able to operate normally before forming of an EL (electro luminescence) element, in an electronic display in which the EL element is formed on a substrate, a manufacturing method of the electronic display including the method of inspecting, a display panel formed with the method of inspecting, and an electronic device comprising the display panel. In particular, the present invention relates to a method of inspecting (inspection method) whether or not the pixel portion is able to operate normally before forming an EL element in an EL display using a semiconductor element (element using a semiconductor thin film), a manufacturing method of the EL display including the method of inspecting, an EL panel formed with the method of inspecting, and an electronic device comprising the EL panel.
2. Description of the Related Art
Techniques of forming a TFT on a substrate have greatly advanced in recent years, and development of applications to active matrix type electronic display devices is proceeding. In particular, TFTs using a polysilicon film have a higher electric field effect mobility (also referred to as mobility) than TFTs using a conventional amorphous silicon film, and therefore it is capable of high speed operation. It therefore becomes possible to perform control of a pixel, conventionally preformed by a driver circuit provided outside the substrate, by a driver circuit formed on the same substrate as the pixel.
The above type of active matrix electronic display device obtains many advantages, such as lowered manufacturing cost, smaller electronic display size, increased yield, and decreased throughput, by building various circuits and elements on the same substrate.
In addition, research on active matrix type EL display devices comprising EL elements as self-light emitting elements has become spirited. The EL display device is also referred to as organic EL display (OELD) or an organic light emitting diode (OLED).
The EL display device is a self-light emitting type, which differs from a liquid crystal display device. The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes, and the EL layer has normally a lamination structure. A lamination structure of “a hole transporting layer, a light emitting layer, and an electron transporting layer” proposed by Tang, et al., of Eastman Kodak Co. can be given as a typical structure. This structure has extremely high light emitting efficiency, and nearly all EL display devices for which research and development is proceeding employ this structure.
Further, additional structures such as “a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer” formed in order on a pixel electrode; or “a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer” formed in order on a pixel electrode may also be used. An element such as a fluorescing pigment may also be doped into the EL layer.
Note that all layers formed between the anode and the cathode are defined as EL layers in this specification. Specifically, layers such as light emitting layers, hole injecting layers, hole transporting layers, electron transporting layers, and electron injecting layers are included in EL layers.
In the above structure, a direct current electric field is generated in the EL layer by application of a predetermined voltage to the EL layer from a pair of electrodes, carrier recombination thus occurs in the light emitting layer, and light is emitted. There is electro luminescence in the EL layer in returning to a base state from a singlet excitation state (fluorescence), and in returning to a base state from a triplet excitation state (phosphorescence), and either type or both types of light emission, may be used.
Note that an element formed by an anode, an EL layer, and a cathode is referred to as an EL element within this specification. Furthermore, the light emission of the EL element is referred to as driving the EL element.
Deterioration of EL layers is promoted by such factors as heat, light, moisture, and oxygen, and therefore EL elements are generally formed after the formation of wirings and TFTs in a pixel portion during the manufacture of an active matrix EL display.
After formation of the EL elements, the substrate on which the EL elements are formed (EL panel) and a cover material are attached and sealed (packaged) by using a sealing material and the like so that the EL elements are not exposed to the atmosphere.
A connector (such as FPC or TAB) is attached after increasing the airtightness by the packaging process or the like in order to connect the elements or terminals leading from circuits, formed on the substrate, to external signal terminals, and the active matrix EL display is complete.
The voltage applied to an EL layer from a pair of electrodes of an EL element in an active matrix EL display is controlled by a TFT formed in each pixel. Therefore, if a TFT in a pixel portion does not function as a switching element, or a wiring is cut or shorted, then a predetermined voltage cannot be applied to the EL layer of the EL element. The pixel cannot display the desired graduation sequence in that case.
If the defect as above develops in the wirings or TFTs for controlling light emission of the EL element, then it is difficult to verify the existence of the defect before the EL display is complete and actual display is performed. Therefore, in order to distinguish an EL panel which will not become a product from a good product, it is necessary to form the EL elements, perform packaging, attach the connectors, and complete the EL display. The processes of forming the EL elements, performing packaging, and connecting the connectors are wasted in this case, and, therefore time and cost for production cannot be reduced. Further, even for EL panels formed by using a substrate which is taken from a multi-faced substrate, the processes of the packaging and the attachment of the connector become wasted, and time and cost similarly cannot be reduced.
With active matrix liquid crystal displays, which have become mass produced ahead of active matrix EL displays, before completing the liquid crystal display by injecting liquid crystals between a panel comprising the pixel portion (liquid crystal panel) and a substrate comprising an opposing electrode, an electric charge is accumulated in a capacitor in each pixel after formation of wirings and TFTs in a pixel portion. Whether or not any defects have developed in the pixel portion is verified by measuring the amount of the electric charge for each pixel.
However, in the case of active matrix EL displays, two or more TFTs are generally formed in each pixel. There are also cases in which one of the electrodes (pixel electrode) of the EL element is connected to a capacitor through a TFT. In this case, even if the amount of electric charge accumulated in the capacitor is measured, it becomes difficult to verify whether or not there are any defects in the wirings and TFTs connected between the capacitor and the pixel electrode.
The establishment of a method of inspection for determining whether or not there are defects in wirings and TFTs in the pixel portion, in other words, whether or not a predetermined voltage can be applied to the pixel electrode of the EL element of each pixel, before completing the EL display, is needed toward mass production of active matrix EL displays.